Schering (No. 3,129,906 Germany) by Gries, Rosenberg, and Weinstien teaches the incorporation of paramagnetic metals into diethylene triamine pentaacetic acid (DTPA) forming chelates useful as contrast agents in magnetic resonance imaging.
Kroll et al (U.S. Pat. No. 3,038,793) teaches the use and manufacture of ethylene diamine diacetic acids containing phenolic groups, of the form N,N'-bis(2-hydroxy-5-Radical benzyl) ethylene diamine diacetic acid.